1. Field of the Invention
The present invention relates to a process for the preparation of a phosphonitrile chloride oligomer mixture having a high content of cyclic phosphonitrile chloride trimer which has a wide range of use and is in greatest demand, for example, as a raw material for the preparation of oligomer or polymer obtained by replacing the chlorine atom of phosphonitrile chloride oligomer or polymer thereof with another group which is excellent in heat resistance, cold resistance, inflammability, electric insulation and the like.
2. Description of the Prior Art
Phosponitrile chloride oligcmer can generally be represented by the formula. EQU (NPCl.sub.2).sub.l (I)
wherein l represents an integer of 3 or greater, and has attracted attention as an industrial material since many of the derivatives and polymers thereof have excellent properties in heat resistance, cold resistance, inflammability, electric insulation and the like. Among the above oligomers, a cyclic phosphonitrile chloride trimer of the above-mentioned formula (I) in which l=3 (hereinafter abbreviated as "3PNC") has a particularly wide range of use and thus is in greatest demand. Accordingly, it has been desired to produce 3PNC with a high yield and a high purity. However, a reaction process for preparing 3PNC alone is not yet known and 3PNC has always been obtained in the form of a mixture with various kinds of oligomers in the conventional production process for phosphonitrile chloride oligomer. Accordingly, it has been attempted so far for the production of 3PNC to improve the reaction yield in the entire mixture of phosphonitrile chloride oligomers as well as increase the ratio of 3PNC formed therein. Several examples of such processes of the prior art are given below. (1) A process for reacting phosphorus pentachloride and ammonium chloride under the presence of a quinoline as the catalyst in a solvent of tetrachloroethane is disclosed in U.S. Pat. No. 2,788,286. Although it has been described therein that 3PNC and the heptamer can be obtained with no substantial formation of the tetramer (hereinafter abbreviated occasionally as "4PNC"), the reaction yield of 3PNC is low in the cited process. (2) Japanese Patent Laid-Open Nos. 3705/1982 and 77012/1982 disclose a process of reacting phosphorus pentachloride and ammonium chloride under the presence of a polyvalent metal compound catalyst, washing a solution of the reaction product in an aliphatic hydrocarbon or an ether with water and recovering a product containing cyclic phosphonitrile chloride oligomers at a high content. Although the yield for 3PNC in this process is relatively high, the process has disadvantages in that the content of 4PNC in the reaction product is high before washing with water and that it takes a long period of time for the reaction. (3) Japanese Patent Publication No. 19604/1983 proposes a process for reacting phosphorus trichloride with chlorine to form phosphorus pentachloride, and reacting the thus formed phosphorus pentachloride with ammonium chloride under the presence of a polyvalent metal compound to produce a phosphonitrile chloride oligomer mixture. However, this publication mentions nothing concerning the individual formation rates for 3PNC and 4PNC. (4) A process for the preparation of phosphonitrile oligomer previously proposed by the inventors of the present invention (U.S. Pat. No. 4,567,028, Japanese Patent Application No. 32525/1984) which comprises reacting phosphorus pentachloride with ammonium chloride in the presence of a catalytic amount of pyridine or its alkyl-substituted derivative and a catalytic amount of a polyvalent metal compound. According to this process, a phosphonitrile chloride oligomer mixture having a high content for 3PNC and a low content for 4PNC which is relatively difficult to isolate from 3PNC and causes trouble in preparing a pure 3PNC can be prepared. For example, a phosphonitrile chloride oligomer mixture comprising at least 65% of 3PNC and less than 10% of 4PNC can be easily prepared.